The transmission medium is often modeled as a non-causal linear channel. In this case, a symbol is received the instant it is transmitted, but it is corrupted by additive noise and a linear combination of past and future data symbols. The effect of past symbols is called postcursor ISI, and the effect of future symbols is called precursor ISI. For causal (and non-causal) channels, a symbol may be received with some delay.
Electromagnetic theory states that the dispersive nature of a channel (i.e. the precursor and postcursor ISI) is unchanged if the roles of transmitter and receiver are exchanged. Reciprocity does not generally hold if the carrier frequency or modulation is changed. This property of reciprocity holds for a limited duration over time-varying channels.